This invention relates generally to data communications over transmission lines, and particularly to determining transmission line characteristics using transmission line devices attached to the transmission line.
Digital transmission techniques are becoming the preferred transmission method of choice for high-speed data/voice/multimedia communications for both business and consumer applications. Digital transmission techniques have been used to obtain 56 kilobits per second (kbps) over plain old telephone service (POTS) lines and a different digital transmission technique is providing data rates exceeding a megabit per second (Mbps) over the same POTS lines.
However, not every arbitrary transmission line will be able to support these high data rates and restrictions on transmission line quality and length are required. For example, in Asymmetric Digital Subscriber Line (ADSL) applications, individual transmission lines (twisted-pair in the case of ADSL) generally cannot be greater than 19,000 feet (19 kft) in length and if bridge taps are present on the transmission line this maximum transmission line length reduces further. A bridge tap is a commonly used way for attaching additional circuits to an existing transmission line. In a home application, bridge taps are used to provide multiple telephone extension lines from a single telephone line coming into the home. The presence of a bridge tap can significantly alter the spectral characteristics of the transmission line. While ADSL installations have been successful with transmission lines with lengths at or near 19 kft and in some cases with bridge taps, the performance is typically quite low and sable operation cannot be assured. Even simple adherence to the transmission line restrictions is not sufficient to ensure a successful installation. This may be due to interference from external sources, such as far-end and near-end crosstalk, AM radio, and in-home appliances with electrical motors.
A typical installation scenario involves an end-user, desiring a high-speed connection to the Internet, placing a telephone call to a local service provider and requesting service. From the end-user""s telephone number and perhaps his address, the local service provider can determine if a high-speed connection is potentially available to the end-user. If the end-user resides in a boundary area (an area where transmission line lengths are approximately equal to the maximum allowed line length) where a definitive YES or NO answer cannot be given, the service provider may perform a simple diagnostic probe of the end-user""s transmission line (for ADSL, the end-user""s telephone line) to determine the end-user""s loop characteristics. Even if the diagnostic probe has determined that a high-speed connection is possible, i.e., the line length is within allowable limits, it does not necessarily mean that a high-speed connection can be established. A system using a transmission line that is at the upper limit of the maximum allowed length is extraordinarily sensitive to external interference and the service provider would not be able to determine functionality until a modem is actually in place at the end-user""s home and a connection attempted. Even an end-user who is using a short transmission line may not be ensured a certain quality of service if his transmission line is of particularly poor quality.
Many techniques have been presented in the past for determining the length and spectral characteristics of a transmission line. One commonly used technique is time domain reflectometry (TDR). In TDR, a test pulse or a test waveform is transmitted down the transmission line and any reflections from an impedance discontinuity or defect in the transmission line along with the time it takes for the pulse to reach the discontinuity and return is recorded. The location of the discontinuity is calculated from the elapsed time and the type and magnitude of the discontinuity is determined from the distortion of the test pulse. TDR is a highly sensitive technique that can reveal not only major defects, such as open or short circuits, but also minute variations, such as impedance variations, frayed shields and bridge taps. TDR however, requires special and usually expensive test equipment and trained operators to properly perform the tests and interpret the results. Additionally, integrating TDR into existing communications devices is very difficult and typically involves a complete device redesign. Hence, time domain reflectometry is usually only performed from the service provider""s end of the connection and not at the end-user""s end. TDR is therefore not a method that the typical end-user can use to self-diagnose problems on his or her high data-rate connection. A need has therefore arisen for a way to diagnose a transmission line without requiring specialized equipment and training.
In one aspect, the present invention provides an apparatus for performing diagnostics on a transmission line, said apparatus comprising a test signal generator coupled to the transmission line for injecting test signals onto the transmission line, a signal calculating circuit with an input coupled to the transmission line for measuring and calculating spectral characteristics of the transmission line, an equivalent line length calculator coupled to the signal measuring circuit for calculating the equivalent line length of the transmission line using the spectral characteristics of the transmission line, a cost function calculator coupled to the signal measuring circuit for calculating a cost function using the spectral characteristics of the transmission line, and a bridge tap detector, coupled to the cost function calculator for determining the presence of a bridge tap by comparing the output of the cost function calculator with a threshold.